bppoim conference rosen external pipeline inspection tool for inaccessible

BPPOIM Conference Asset Integrity Management, Inspection and Cathodic Protection

October 2013 P a g e | 1 External Inspection Tool for Inaccessible Areas and Pipe Supports

External Inspection Tool for Inaccessible Areas and Pipe Supports

Mark F. Rosa1, Ali Minachi1, Nowmaan Anwar2, Mashaan M. Al-Shammari1, Saeed O. Al-Malki1, Saeed F. Farea1, Rami G. Hammad1

1Saudi Aramco, Dhahran, Saudi Arabia 2Rosen Middle East, KFUPM Techno Valley, Dhahran, Saudi Arabia

ABSTRACT: When internal scraping of pipelines is not possible, alternative inspection solutions are often inadequate. Recently, Saudi Aramco has tested a new set of electromagnetic acoustic transducer (EMAT) tools that improve upon available external inspection methods for pipes. These tools work in tandem; the first sending axial ultrasound down the length of the pipe providing pulse-echo data of indications and their positions up to several meters from the transducer. The complimentary tool then sizes the detected indications. Using higher-order guided waves, these tools are able to resolve reflections from smooth corrosion whereas most other techniques fail without a sharp feature. Corrosion appears in various forms, including gradual and smooth, and the ability of these tools to detect this elusive corrosion sets them apart from other techniques. In addition, these tools are uniquely adapted for imaging defects in inaccessible areas such as soil-air interfaces and under pipe supports. A case study of the tool’s development and validation at Saudi Aramco will be presented and findings will be shared.

INTRODUCTION: The advanced nondestructive testing (NDT) tools used in this study are among a class of in-field service equipment that utilize guided wave electromagnetic acoustic transducers (EMATs) to detect and size metal loss anomalies and have high probability of detection (POD) of crack or crack-like features. The first tool utilizes EMAT sensors that transmit and receive ultrasonic waves in the axial or longitudinal direction through pulse-eco data acquisition (Figure 1). The sensor can be unidirectional or bi-directional. This type of tool is normally used for screening tasks; however, the sizing of features is also possible with a case-by-case calibration procedure or with the use of the so-called acoustic feeler gauge, which provides corrosion depth information with remote corrosion images. It brackets the remaining wall thickness of detected features through the use of distinct wave modes applied to each feature. These guided waves reflect both from thinning beyond a certain depth, as well as from a feature’s edges, which provide strong indication signals. In pipes, these tools can inspect with a confidence of 80% at



2 m distance, depending on reflected signal strength which is affected by type and quality of defect and presence of any wave scattering. Practically, an inspector can comfortably inspect within the 1-2 m range on pipes. With or without the feeler gauge, the axial EMAT is capable of realizing reflections from smooth corrosion features where most other techniques fail due to the lack of a sharp geometrical reflector. Corrosion presents itself in various shapes and sizes, and can often be gradual and smooth. The ability to pick up a signal from smooth thinning is a key feature, which makes this inspection unique from other techniques [1,2]. In addition, this tool is further specialized for the inspection of structures that are not easily accessible such as soil-to-air interfaces, pipe supports, pipes under thrust anchors, ring girders and hard-to-reach areas on tanks and vessels, among other such areas. Pipes under sleeves can also be inspected with the limitation that the high reflection from the sleeve can hide possible defects.

Figure 1: Axial tool. The left image shows an EMAT sensor tool in operation on pipe with propagation of induced ultrasonic waves as illustrated in the right hand image. Waves reflecting off indications are detected by the transducers in a pulse-echo process.

The principle behind the EMAT technique itself is an alternative method to inducing ultrasonic waves in a solid material. The more commonly used method for inducing ultrasonic waves in a material is by means of direct coupling — be it by a complete liquid immersion method or by utilizing a thin layer of liquid for couplant between the piezoelectric probe and the material of interest — thereby minimizing microstructure air gaps between the adjacent surfaces, which would otherwise prevent significant wave propagation in the bulk material of interest. In contrast, EMATs do not require any couplant as they operate on the basis of Lorentz forces acting on eddy currents induced in the material of interest according to the equation [3]:

𝐅 = 𝑞(𝐄 + 𝐯 × 𝐁) where F is the force on a point charge q, v is the velocity of that charge, E is the induced electric field and B is the magnetic field. The EMATs operate by inducing moving charges subject to electric fields in the material as eddy currents, and imparting the Lorrentz force on these currents with the use of the applied static magnetic field. The benefit of EMAT ultrasound generation is that it does not require contact with the surface and so has special applications such as at high temperature, as well as added benefits due to the lack of couplant including



increased speeds and greater flexibility of operation. One drawback of EMATs, in practice, is that they require strong magnetic fields and large currents to generate their ultrasonic waves due to the fact that wave generation in the material of interest is generally weaker than that produced by piezoelectric crystals [3,4]. Another drawback is that EMAT data interpretation requires fine tuning for proper analysis and consequently necessitates an experienced and trained technician.

In the course of this study we sought to determine whether the EMAT tools performed as reported in actual field conditions. These reported specifications are included in Attachment I.

EXPERIMENTAL SETUP AND MEASUREMENT TECHNIQUES: The EMAT inspection was performed in a blind test on a sample pipe of 24” diameter, 0.375 wall thickness and 179 in. (4540 mm) long. Saudi Aramco chose a pipe for this study with known defects, both from service and manufactured as such. These defects were underneath a 1 m long sleeve and so were not visible to the EMAT inspector during the trials. This setup is shown in Figure 2. The measurement techniques consisted of utilizing the EMAT tools on both sides of the sleeve. As can be seen in Figure 2, the end of the pipe closest to the sleeve was only approximately 375 mm long after the termination of the sleeve, which made scanning difficult due to the large back-echo resulting from the terminated end of the pipe. This back-echo effectively masked any indications beneath the sleeve for data collected from this side. As a result, the upstream side of the sleeve proved to be the only reliable side on which data was collected. It should be noted that in actual practice this would not be an issue as pipeline usually continues rather than terminates in this manner. Therefore, in these cases, as seen by the EMAT tool, the pipeline continues indefinitely on each side of the area of interest.

Figure 2: The 24 in. pipe sample with simulated artificial defects. The left image shows the zero reference designated as the 12 o’clock position at the end opposite the sleeve; the sign convention of the pipe’s axis follows the right hand rule as oriented by the indicated direction of pipe flow. The right image shows the sleeve.

A representative sample of defects included in the pipe showing the extent of wall loss, defect dimensions and a qualitative description of the actual defects in the test pipe are presented in Table 1. The actual layout of the defects in the pipe is presented in Figure 3. This representative



sample of defects demonstrates the wide range of defect types and severity included in the pipe. The goal of selecting a pipe with such a wide range of defect characteristics was to test the full capabilities of the EMAT tools with regard to the types of defects they are capable of detecting and characterizing.

Table 1: Representative Sample and Description of Actual Defects in the Sample Pipe Wall Loss (%) Length (mm) Width (mm) Indication Description

15% 40 50 Thinning 16% 30 50 Thinning 29% 10 10 Pitting 32% 90 10 Gouge Axial 35% 80 10 Gouge Axial 35% 105 60 Thinning 45% 70 50 Thinning 48% 10 70 Gouge (Circumferential) 51% 10 10 Pitting

100% 10 10 Hole

Figure 3: Actual layout of defects in test pipe.



FIELD TRIAL RESULTS: After the blind test, a truth table describing the actual indications in the pipe was compared to the indications found by the EMAT examination. A comparison of these results is presented in Figure 4, which shows both data sets on the same plot: the indications were color-coded based on defect depth. The squares represent the actual features and are labeled with the prefix “A;” while the triangles represent the EMAT findings and are labeled with the prefix “S.” Each indication is also labeled with a corresponding number for reference.

Figure 4: EMAT results on 24 in. test pipe with a 0.375 mm wall thickness.

A3

A4

A7

A10 A13

A14

A15

A6 A8

A17

A18 A19

A1

A9

A11

A2 A5

A12

S1

S3

S15

S6

S19

S21

S13

S20

S25

S14

S30

S2

S11

S5

S33 S34

A16

0

250

500

750

1000

1250

1500

1750

2000

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Circ

umfe

rent

ial P

ositi

on (m

m)

Absolute Axial Position (mm)



The graph shows all the indications at the reported locations. Sources of error in this data, as deduced by the inspector, arise from the following common sources of error for such EMAT trials: (1) beam spread, (2) encoder accuracy and (3) general data collection inaccuracy. Despite this minor error, the correlation between the actual and detected indications was satisfactory. The graph groups the features in circles, bounding related indication groups to illustrate each of the major indications and their corresponding EMAT detection data.

ANALYSIS: The EMAT tools reported each indication close to the actual position and size. In particular, the tools performed well considering the wide variation of shape and orientation of the features. Discussion of representative defects is below.

Figure 5: Representative defects in sample 24 in. 0.375 mm wall thickness pipe.

As can be seen in the plot in Figure 4 (with the actual defects displayed in Figure 5), feature S2 corresponds to two actual defects, A1 and A2. This demonstrates that the EMAT tools could not distinguish both flaws due to their close proximity to each other. Nonetheless, sizing was accurate. A lack of resolution of this sort of close defect pair can be considered acceptable as the tools would be able to find either defect if alone; and this is more important in the oil and gas industry. Moreover, depending on the maintenance standard used, some defects in close proximity can actually be considered as one single defect anyway.

Similarly, as seen in the plot in Figure 4 and displayed in Figure 5, features S13, S14 and S15 correspond to A5, A6 and A8, respectively. Here the locations were accurate but sizing was difficult according to the EMAT inspector, due to the orientation of the defects being in line with the direction of signal propagation, which provided less of a reflection on which to base judgment. Qualitatively, the sizing was correct for the group in terms of severity ranking, despite exact wall loss percentages being off.

A7

A8

A6

A1

A2



Mode Conversion: In addition to the actual defects reported in the sample pipe, the EMAT tools also reported additional indications. As can be seen in Figure 4, the EMAT scan data clearly showed indications (features S19, S20, S21 and S25) around the 6 o’clock position, which had no corresponding actual defect, “A.” As a result, this section of the sample pipe required further study to determine whether the EMAT tools were reporting incorrectly or if they had in fact detected additional defects beyond what was previously known in the sample pipe.

Further study yielded the results displayed in Figure 6 and Figure 7. These results show that indication S19 and S20 appear to originate from the same feature. When viewed at close range they appear to be two separate and close reflections (illustrated in the left hand image of Figure 6); but these two reflections separate in time when a longer path length is used (illustrated in the right hand image of Figure 6). This behavior demonstrates one of the characteristics of mode conversion, a common phenomenon arising in guided wave analysis. After a guided wave is reflected from certain geometries, the reflection can transmute into separate and distinct wave modes with different group velocities. The differing group velocities affect the arrival times of the various modes. As a result, they arrive at different times, or distances, when plotted with distance. With regard to defects, this mode conversion is particularly likely to occur from a feature with a specific, nontrivial axial length, which produces complimentary wavelets from its leading and trailing edges.



Figure 6: Features S19, S20, S21 and S25 displayed in two successive scans. The left hand image at close range and right hand image at a further distance. The mode conversion indicates that S19 and S20 are the same feature, evidenced by the arrival time shift between these two successive scans.

In addition, mode conversion can also occur if the feature is a particularly effective reflector, regardless of length or exact geometry. Such an example from this study is shown in Figure 7, which illustrates the behavior of the efficient reflector, A9 (S11). This feature is a circumferential groove, shown physically in the bottom image of Figure 7. The top two images of Figure 7 show the reflection of this feature, from both directions: the top left scan shows the downstream-incident reflection 620 mm away from the transducer while the top right scan shows the upstream-incident behavior at 770 mm away. Some key features can be discerned from comparison of these two scans: the top left image appears as three closely grouped echo peaks, which are likely interference patterns giving rise to three peaks. The top right scan image shows similar closely grouped reflections but also has a late-arriving slower reflection approximately 200 mm away from the main echo. This later echo was likely from a slower converted mode rather than from an additional feature; this mode was probably the S0 mode, which has a relatively low group velocity [5].



Figure 7: The top left image shows original scan data of circumferential notch A9 (S11) (photo bottom image); three echo peaks result from this single indication 620 mm away from the transducer. The top right image shows the same indication from the opposite direction; 770 mm from the transducer with a slower mode arriving approximately 200 mm after.

Strengths and Limitations: Shadowing, or the masking of one feature by another nearby feature that lies in the direction of the EMAT wave propagation, is a reasonable concern with any guided wave inspection method, especially when the features have different depths or dimensions. With these tools, the shadowing of larger features by smaller features does not affect the results significantly as the larger feature will reflect most of the acoustic energy and yield the dominant indication. Larger features can shadow smaller features since larger features reflect most of the energy, leaving little remaining for the smaller indication in its shadow. Generally, this latter case is not a problem because such smaller features are less important than the bigger ones, which are not affected. An extension of this is detectability of a feature that is in middle of two other features falling in the same line. In this case, again, the largest feature will dominate especially if it is the



first in line. If a large feature is surrounded by smaller features, it would dominate. The smaller, closer, indication might appear but the last and further feature would almost certainly be lost to the other closer features. Moreover, it depends on the shape and size of the feature. The circumferential groove, A9, which was perpendicular to the sound path was well shaped to reflect and consequently mask all other indications around it. If the features are all similar in shape, such as a large gentle corrosion area, the deepest area would cause the largest echo and, therefore, make the largest indication despite the surrounding shallow area. This is the benefit of the bracketing ability of the acoustic feeler gauge concept: only corrosion deeper than a given value will reflect and make an indication representative of its shape at that particular depth.

Another limitation pertains to coatings. The effects of common coatings in the oil and gas industry, such as tape wrap, composite sleeves, epoxies and paint, must be considered. As with most other guided wave techniques, generally the thicker and more adhesive the coating, the more it will limit inspection distances. For example, with these EMATs, a tar coating could limit inspection distances to 300 mm.

Detecting features under a welded sleeve is also problematic because the weld would produce a large echo and so significantly reduce the energy available to propagate into the area of interest. If the sleeve was of a significantly different thickness than the parent pipe, the guided wave would be more likely to continue propagating in the original pipe; however, safety usually requires the sleeve to be of comparable size to the parent pipe and so welded sleeves are generally not well suited for inspection beyond the weld by these EMAT technologies.

Recommendations: In addition to the axial EMAT tools used in this study, it is recommended that test engineers also explore the use of other available complimentary EMAT technologies. One complimentary technology, which has been used previously in a Saudi Aramco field trial and shown to produce promising results, consists of EMAT sensors transmitting ultrasonic waves in the pipe’s circumferential direction (Figure 8). It should be noted that the sensors must locally be in contact with the bare pipe, however away from the sensors the pipe may still be coated. Hence, coating removal is only necessary at the exact locations where the sensors will be applied. Normally with this configuration, two EMAT sensors are applied to the structure resulting in a long-range and short-range ultrasonic wave. As a result, only two transducers are required to inspect the full body of the pipe; in large diameter pipes, there is a wider spacing of the sensors and the transmitting sensor generates higher power signals for greater reach of the induced waves, however still only two transducers are necessary. Adding more than two transducers will add more resolution to the dataset. For example, two transducers provide two pixels of resolution: a 1/3 data point and a 2/3 pipe data point along the length of the pipe; three



transducers would provide three pixels. The tool is adapted for sizing of features and can be used for any pipe inspection, including the inspection of pipes on supports. Moreover, the tool acts in a complimentary manner to the axial EMAT tool with its capability to size the defects.

Figure 8: Circumferential tool. The left image shows an application of a tool with a two-EMAT sensor configuration. The possible resulting ultrasound wave paths are indicated in the right image.

CONCLUSIONS: The results of this trial demonstrated promising results for these newly-developed EMAT tools. The chosen test pipe, which was less than optimal due to its length and the positioning of the sleeve, limited the inspection capabilities of the tools. Despite these limitations, the tools in this study were still able to produce location and sizing data for most actual defects to reliable accuracies. As a result the investigating team determined that these tools can provide added benefit to inspection programs in the oil and gas industry. It should be noted that, as with all inspection technologies, these EMAT tools are not without limitation and their use should be applied accordingly. Some of the limitations include inadequate detection of certain defects in a linear configuration due to the shadowing effect, data analysis challenges such as mode conversion anomalies, and reduced inspectability of certain equipment with the presence of thick coatings and welded sleeves.



REFERENCES:

1. A. Cosham, P. Hopkins, “The Assessment of Corrosion in Pipelines – Guidance in the Pipeline Defect Assessment Manual (PDAM),” in Pipeline Pigging and Integrity Management Conference, Amsterdam, The Netherlands, May 17-18, 2004.

2. P. Hopkins, “The Structural Integrity of Oil and Gas Transmission Pipelines,” in Comprehensive Structural Integrity, 1, final draft for editor, pp. 1-62 (2002).

3. C. Edwards, S. Dixon, A. Widdowson and S. B. Palmer, “Electromagnetic Acoustic Transducers for Wall Thickness Applications in the Petrochemical Industry,” in Review of Progress in Quantitative Nondestructive Evaluation, edited by D. O. Thompson, D. E. Chimenti, pp. 1973-1800 (2000).

4. F. Kojima, D. Kosaka and K. Umetani, “Continuous Surveillance Technique for Flow Accelerated Corrosion of Pipe Wall Using Electromagnetic Acoustic Transducer,” in Review of Progress in Quantitative Nondestructive Evaluation, 30, pp. 1341-1346 (2011).

5. D. Greve, J. Neumann, J. Nieuwenhuis, I. Oppenheim, N. Tyson, “Use of Lamb Waves to Monitor Plates: Experiments and Simulations,” in SPIE Proceedings, Smart Structures and Materials, 5765, edited by M. Tomizuka, pp. 281-292 (2005).



ATTACHMENT I

TECHNICAL SPECIFICATIONS

Standard Operating Specifications: Axial Tool Circumferential Tool

Diameter range > 3 in. 4-64 in. (plates)1 Ambient temperature 0 °C to 50 °C (32 to 122 °F) 0 °C to 50 °C (32 to 122 °F) Sensor temperature -20 °C to 100 °C (-4 to 212 °F) -20 °C to 100 °C (-4 to 212 °F)

Speed of measurement < 12.5 mm/s (< 0.04 ft/s) < 12.5 mm/s (< 0.04 ft/s) Wall thickness range 3-15 mm (0.12-0.59 in.) 3-15 mm (0.12-0.59")

Coating thickness < 0.5 mm (< 0.02 in.) < 3 mm (< 0.12") Dead zone from scanner 10 cm (3.94 in.) 15 cm (5.91") Pipe on support per day2 10 10

Pipe body per day2 n/a 500 m (0.310 miles) 1 > 14 in. multiply scans, specification as short path, valid up to plates 2 Good access and clean pipeline

DETECTION CAPABILITIES Axial Tool

Feature Detection threshold at 300 mm (11.8 in.)

Detection threshold at 600 mm (23.6 in.)

Pitting 0.40 t 0.60 t Thinning 0.40 t 0.40 t

Cracks High defect POD (depends on factors i.e. orientation, etc.)

High defect POD (depends on factors i.e. orientation, etc.)

Notes: • Probability of detection (POD) at 80% certainty; no coating; clean surface • Circumferential tool measures direct transmission signal; axial tool measures pulse echo signal • Sizing come in categories based on corrosion models (0-20%, 20-40%, 40-60%, > 60% no accuracy value) • Pitting diameter: Diameter = 3 × depth (3:1 aspect ratio) • Axial notches: 50 mm length (2 in.) • Thinning diameter: 50 mm × 50 mm (2 × 2 in.)

Circumferential Tool Feature Detection threshold Pitting 0.30 t Notch 0.20 t

Thinning 0.15 t

Cracks Generally high POD, dependent on factors such as orientation.

• Notes: POD at 80 % certainty; no coating; clean surface; no seamless

Abbreviations: t = wall thickness

bppoim conference rosen external pipeline inspection tool for inaccessible

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